Multi-view display device

ABSTRACT

The invention provides a multi-view display in which a view forming arrangement comprises a first view forming structure spaced by a first distance from the display panel for providing multiple views across a first direction, and a second view forming arrangement spaced by a second distance from the display panel for providing multiple views across a second perpendicular direction. The angular width of the multiple views in the two directions can thus be independently defined.

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application a continuation of U.S. application Ser. No. 14/895,072filed Dec. 1, 2015 which claims the benefit of International ApplicationNo. PCT/EP2014/060469 filed on May 21, 2014, which claims the benefit ofEuropean Patent Application No. 13170243.3 filed Jun. 3, 2013. Theseapplications are hereby incorporated by reference herein.

FIELD OF THE INVENTION

This invention relates to multi-view displays.

BACKGROUND OF THE INVENTION

A multi-view display is typically created by applying a special layer toa 2D display. Known options for this layer are a barrier for barrierdisplays, a lenticular lens sheet for lenticular displays or amicroarray of lenses.

No matter which option is chosen, the effect is that depending on theviewpoint of an eye (or camera) a different image is projected, thusproviding stereoscopic vision (stereopsis) without needing specialglasses. This is what is meant by “auto” stereoscopic.

FIG. 1 shows the basic principle for a display using a lenticular lensarray. The display comprises a conventional (2D) display panel 2 havingan array of pixels 4 over which a view forming arrangement 6 isprovided. This comprises lenticular lenses 8. If each lens overlies 4pixels in the display width direction, then light from those four pixelswill be projected in different directions, thereby defining differentviewing areas, numbered V1 to V4 in FIG. 2. In each of these viewingareas, an image is projected which is formed as the combination of allpixels with the same relative position with respect to the lenses.

The same effect can be achieved with barriers, which limit the outputdirection with which light is emitted from each pixel. Thus, in eachoutput direction, a different set of pixels can be viewed.

The increase in angular resolution (i.e. the multiple views) results ina diminishing of the spatial resolution (i.e. the resolution of eachindividual view). In the case of vertical lenticular sheets andbarriers, this resolution reduction is entirely in the horizontaldirection. By slanting the lenticular sheet the resolution reduction canbe spread over both horizontal and vertical directions providing for abetter picture quality.

FIGS. 2 and 3 show examples of 3D lenticular display constructions.

FIG. 2 shows the least complicated design, comprising a lenticular lenssheet 6 over the display panel, with a spacer 10 between. The curvedfaces of the lenticular lenses face outwardly, so that convex lenses aredefined.

FIG. 3 shows a preferred design which has better performance under wideviewing angles. The curved lens surfaces face the display panel, and areplica layer 12 is used to define a planar internal surface. Thisreplica can be a glue (typically a polymer) that has a refractive indexthat is different from that of the lenticular lens, so that the lensfunction is defined by the refractive index difference between the lensmaterial and the replica material. A glass or polycarbonate slab is usedas the spacer 10, and the thickness is designed to provide a suitabledistance for the lenticular lens to focus on the display panel.Preferably the refractive index of the slab is similar to the refractiveindex of the glue. It is well known that a 2D/multi-view switchabledisplay can be desirable.

By making the lens of a multi-view display electrically switchable, itbecomes possible for example to have a high 2D resolution mode (with nolens function) in combination with a 3D mode. Other uses of switchablelenses are to increase the number of views time-sequentially asdisclosed in WO 2007/072330 or to allow multiple 3D modes WO2007/072289.

The known method to produce a 2D/3D switchable display is to replace thelenticular lens by a lens-shaped cavity filled with liquid crystalmaterial. The lens function can be turned on/off either by electrodesthat control the orientation of LC molecules or else by changing thepolarization of the light (for example using a switchable retarder). Theuse of graded refractive index lenses has also been proposed, in which abox-shaped cavity is filled with liquid crystal and an electrode arraycontrols the orientation of LC molecules to create a gradient-index lens(this is disclosed for example in WO 2007/072330). An electrowettinglens, which is formed of droplets of which the shape is controlled by anelectric field has also been proposed for 2D/3D switching. Finally, theuse of electrophoretic lenses has also been proposed, for example in WO2008/032248.

As mentioned above, there is always a trade-off between spatial andangular resolution. Displays with lenticular lenses and verticalbarriers offer horizontal parallax only, allowing for stereopsis andhorizontal motion parallax and occlusion, but not vertical motionparallax and occlusion. As a result, the autostereoscopic function ismatched to the orientation of the display. Only with full (horizontaland vertical) parallax can the 3D effect be made independent of thescreen orientation.

However, at least in the medium term, display panels will not havesufficient resolution to enable full parallax at HD resolution, at leastnot with large numbers of views. There is therefore a problem fordevices that are designed to operate in portrait and landscape mode,such as handheld devices.

This problem has been recognized, and some of the solutions above whichprovide 2D/3D switching capability have been extended to includemultiple 3D modes, such as portrait and landscape modes. In this way,three modes are enabled: 2D, 3D portrait and 3D landscape.

Full parallax may be possible already for a system comprising just twoviews, thus resulting in only moderate resolution loss and therefore theswitching between 3D modes can be avoided. If a non-switching approachis to be used, the minimal microlens array design that is dual view anddual orientation has 2×2 views and preserves the maximum amount ofspatial resolution.

The common RGB stripe pixel layout comprises red, green and bluesub-pixel columns. Each sub-pixel has an aspect ratio of 1:3 so thateach pixel triplet has a 1:1 aspect ratio. The lens system typicaltranslates such rectangular 2D sub-pixels into rectangular 3D pixels.

When a microlens is associated with such a display panel, for examplewith each microlens over a 2×2 sub-array of pixels, the lens design hasthe problem that the viewing cone in one of the two orthogonaldirections is three times as wide as in the other.

FIG. 4 shows this effect, and shows each group of 4 sub-pixels withthree pixels on and one off, and with a 10% defocus. This means thefocal length of the lenses differs by 10% compared to the lens-displaydistance. This is to prevent sharp focusing of the black mask patternbetween the pixels.

The peaks in the light intensity plots show the positions of therepeated views (i.e. within different viewing cones) of a given pixel.They show the light power per unit area (in Watts per mm²) at differentpositions across the display screen. One plot is for the landscape modeand the other is for the portrait mode. Thus, the pitch of the repeatingpattern corresponds to the viewing cone width. Clearly, in the directionacross the long sub-pixel axis (the x-axis), the viewing cone width ismuch larger than in the direction across the short sub-pixel axis (they-axis).

The bright areas represent illuminance distributions from each group of3 pixels turned on, on a plane situated at the optimum observationdistance from the display. The x- and y-axes represent lineardisplacements.

Small viewing cone angles have a tangent which may be approximated bythe lenticular pitch divided by the thickness of the stack. For the RGBstripes layout, the lenticular pitch in one direction is three times asmuch as in the other as can be seen from FIG. 4, so the viewing conewill be three times as wide as well. As a consequence, at certain(fixed) viewing distance, in one direction (e.g. portrait) the user hasto hold the device carefully to avoid getting out of the cone, while forthe other direction it may be difficult to find the 3D zone because theviews are so wide. There is therefore a need for a full parallaxautostereoscopic display, which enables the viewing cone sizes in thetwo orthogonal display orientations to be independently defined.

US 2013/0069938 discloses a display unit which in one example has twoorthogonal lenticular arrays.

SUMMARY OF THE INVENTION

The invention is defined by the claims.

According to the invention, there is provided a multi-view display,comprising: a display panel; and a view forming arrangement formed overthe display panel for providing a multi-view function, wherein the viewforming arrangement comprises a first view forming structure spaced by afirst distance from the display panel for providing multiple viewsacross a first direction, and a second view forming structure spaced bya second distance from the first view forming structure for providingmultiple views across a second perpendicular direction, such that theangular width of the multiple views in the two directions isindependently defined, wherein the angular widths of the multiple viewsin the two directions are in the ratio of smaller angular width tolarger angular width of 1:n where n<2. This arrangement separates theprovision of multiple views across the display between two view formingstructures, each for different orthogonal directions. Together, theyprovide full parallax, so that the display can be viewed in portrait orlandscape mode without requiring any switching function. Preferably,n<1.5, even more preferably n<1.2.

These angular widths thus differ by less than 100% (i.e. the larger isno more than double the smaller), and more preferably even less, forexample less than 50% (the larger is no more than 1.5 times the smaller)or even less than 20% (the larger is no more than 1.2 times thesmaller). This makes the viewing cones of similar size. By “angularwidth of the multiple views” is meant the angle over which one full setof views is displayed along one of the viewing directions. Itcorresponds to the angle over which a set of pixels corresponding to theset of unique views in one of the viewing directions can be viewedthrough a single view forming element (i.e. lens or barrier opening). Ata more remote viewing angle these pixels becomes visible through anadjacent view forming element.

Preferably, both view forming structures are operable at the same timeso that no switching between modes is needed. The display light passesthrough both view forming structures. One provides parallax in onedirection and the other provides parallax in the other direction. Thus,the display can be rotated between orientations without needing anyswitching of the display configuration. However, one or both of the viewforming structures can be made electrically switchable, in known manner.

By providing the same angular width (which is often termed the conewidth) in the two orthogonal directions the optical performance can bematched in the different orientations. In order to provide thismatching, the spacing distances as well as the materials used in thestack (such as the spacer materials) can be selected. If materials ofthe same refractive index are used, the design simplifies with only thegeometric distance needing to be taken into account. The size of theview forming elements (lenses or barriers—which together make up theview forming structures) is typically dictated by the underlying pixelconfiguration, since each individual view forming element is intended tooverly a certain number of sub-pixels of the display, which thendetermines the number of views to be formed.

The display panel may comprise rectangular sub-pixels. Rectangularpixels give rise to the viewing cone variation in different orientationswhen microlenses are used. The aspect ratio of the sub-pixels can be1:3, which is typically the case for RGB striped pixel configurations.

The first view forming structure preferably has a periodic structure,with a period based on the number of sub-pixel dimensions in a firstdirection across the sub-pixels (but the period is corrected to providefocusing to the desired viewing distance), and the second view formingstructure has a periodic structure, with a period based on the number ofsub-pixel dimensions in a second orthogonal direction across thesub-pixels (but again the period is corrected to provide focusing to thedesired viewing distance).

If the period for each view forming structure is based on the number ofsub-pixels numbers as mentioned above, it means that the same number ofviews is generated in the landscape and portrait modes, by providing thesame number of sub-pixels per view forming element. When the underlyingsub-pixels are rectangular, this results in different required pitch forthe two view forming arrangements.

The periods for the two view forming structures can be based ondifferent numbers of sub-pixels for portrait and landscape modes. Thiswill result in different resolution loss in the two orientations but canstill correct for different viewing cone sizes.

The first view forming structure closest to the display panel can bemade of material with a first refractive index n, and the second viewforming structure can be made of material with a smaller refractiveindex. This arrangement enables the thickness of the optical stack to bekept to a minimum.

In a preferred example,

$\frac{p_{1}}{\left( {t_{1}/n_{1}} \right)} = {k \cdot \frac{p_{2}}{\left( {t_{1}/n_{1}} \right) + \left( {t_{2}/n_{2}} \right)}}$

in which p₁ is the period of the first view forming structure, t₁ is theheight of the first view forming structure over the display panel and n₁is the refractive index of the material between the display panel andthe first view forming structure, p₂ is the period of the second viewforming structure, t₂ is the height of the second view forming structureover the first view forming structure, and n₂ is the refractive index ofthe material between the first and second view forming structures,wherein k is between 0.5 and 2, more preferably between 0.75 and 1.5,more preferably between 0.9 and 1.1.

This means the ratio of the period of the one view forming structure toan effective optical distance (distance divided by refractive index) ofthe one view forming structure from the display panel and the ratio ofthe period of the other view forming structure to an effective opticaldistance (distance divided by refractive index) of the other viewforming structure from the display panel are made to be similar. Thisresults in the viewing cones being substantially the same. The ratioscan of course be equal (k=1).

This equation simplifies to geometric distances only if the refractiveindex values are the same.

In one set of examples, the view forming arrangement comprises a firstspacer layer over the display panel, a first lens layer (e.g. lenticularlens array) over the first spacer layer, a second spacer layer over thefirst lens layer and a second lens layer (e.g. lenticular lens array)over the second spacer layer.

The spacer sizes and materials enable control over the viewing coneangles. The first and second lens layers can define convex lensinterface shapes, with respect to the direction of light through theview forming arrangement from the display panel. In this case, the firstspacer layer, the first lens layer and the second lens layer can beglass or plastic, and the second spacer layer is air.

In another example, the first lens layer defines convex lens interfaceshapes, and the second lens layer defines concave lens interface shapes,with respect to the direction of light through the view formingarrangement from the display panel. In this case, the first spacerlayer, the first lens layer and the second lens layer can be glass orplastic with a first refractive index, and the second spacer layer isglass or plastic with a second, lower refractive index.

In an alternative set of examples, the view forming arrangement cancomprise a first spacer layer over the display panel, a first barrierlayer over the first spacer layer, a second spacer layer over the firstbarrier layer and a second barrier layer over the second spacer layer.The invention can thus be applied to barrier type displays as well as tolenticular lens type displays.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples of the invention will now be described in detail with referenceto the accompanying drawings, in which:

FIG. 1 shows a known multi-view display to explain the basic principleof operation;

FIG. 2 shows a first example of known lens design;

FIG. 3 shows a second example of known lens design;

FIG. 4 is used to explain the problem of different viewing cone sizesfor different display orientations;

FIG. 5 shows a first example of view forming arrangement of theinvention;

FIG. 6 shows how the problem of different viewing cone sizes fordifferent display orientations is resolved by the design of FIG. 5;

FIG. 7 shows a second example of view forming arrangement of theinvention; and

FIG. 8 shows a third example of view forming arrangement of theinvention based on barriers instead of lenses.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention provides a multi-view display in which a view formingarrangement comprises a first view forming structure spaced by a firstdistance from the display panel for providing multiple views across afirst direction, and a second view forming structure spaced by a seconddistance from the display panel for providing multiple views across asecond perpendicular direction. The angular width of the multiple viewsin the two directions can thus be independently defined.

A regular microlens display does not allow independent design of theviewing cone in first and second directions. In fact, the viewing coneratio equals the sub-pixel aspect ratio multiplied by the ratio ofnumber of views along the two directions:

$\frac{a_{p}}{a_{l}}\frac{N_{P}}{N_{l}}$

where a_(p) and a_(l) are the sub-pixel dimensions along the twodirections (for instance portrait and landscape).

A regular microlens is suitable when

$\frac{a_{p}}{a_{l}}\frac{N_{P}}{N_{l}}$

is close to the desirable viewing cone ratio.

The invention provides a display that performs like a microlens display,but does allow independent design of the viewing cones.

FIG. 5 shows a first example of view forming arrangement of theinvention in the form of a lens stack.

The lens arrangement comprises a first lens arrangement 20 spaced fromthe surface of the display panel 2 by a bottom spacer 22. The first lensarrangement and spacer have a combined thickness of t1 so that the lenssurfaces are a distance t1 from the display panel 2. A second lensarrangement 24 is spaced from the first lens arrangement 20 by a secondspacer 26. The second lens arrangement and the second spacer have acombined thickness of t2 so that the lens surfaces are a distance t2from the first lens arrangement and at a distance of t1+t2 from thedisplay panel 2. The two lens arrangements are designed with sufficientfocus on the pixels in the display panel module.

For thin lenses, the thickness of the lens array can be ignored. Theviewing cone half-angle θ1 in the material of the spacer in the firstdirection as implemented by the first lens array 22 is given by tanθ1=p1/2t1, as can be seen from FIG. 5.

As an approximation, if the viewing cone angle is small, the fullviewing cone angle in the material α1=2θ1 can be approximated by tanα1=p1/t1.

For the example of the two spacers having the same refractive index, theviewing cone half-angle in the material of the spacer in seconddirection as implemented by the second lens arrangement is given by tanθ2=p2/2(t1+t2), or as an approximation for the full viewing cone tanα2=p2/(t1+t2).

If, for example, viewing cones should be designed to be similar, then:

$\frac{p_{1}}{t_{1}} \approx \frac{p_{2}}{t_{1} + t_{2}}$

In the case the two spacers are made of materials with differentrefractive indices and in the approximation of thin lenses, the abovecondition of having similar viewing cones in two directions ofobservation in air can be written as

$\frac{p_{1}}{\left( {t_{1}/n_{1}} \right)} \approx \frac{p_{2}}{\left( {t_{1}/n_{1}} \right) + \left( {t_{2}/n_{2}} \right)}$

where n₁ and n₂ are refractive indices of the material of the first andthe second spacer respectively.

This equation takes account of the refractive index values in the stack.If the refractive index n₁=n₂, then the second equation simplifies tothe first, and only the geometric distance needs to be taken intoaccount. The refractive index of the lenses also need to be taken intoaccount for a complete optical analysis, although typically the spacersare thicker than the lenses so that the spacers dominate.

The reason why a value t/n is required when taking account of therefractive index values is that the cone angles are calculated in themedium, but the effective 3D cone angles which the user perceives are inair.

According to Snell's law n×sin(α_(n))=sin(α_(air)) Using theapproximation for small angles:

n×p/t=p/t _(effective) so that t _(effective) =t/n.

For example, in the case of an RGB striped display, where the pixelcomponents have a height to width ratio of 3:1, for the design with thesame number of views in two observation directions (for instance 2×2view design) the pitches of the lens-stack relate as 3p₁=p₂, so 2t₁≈t₂

This means the spacer that is sandwiched by the lenses is opticallythicker than the spacer between the display panel and the first lens 20.

The lens design of the invention can use non-switchable lenses, so thatfull parallax is provided permanently. The same viewing cone performanceis obtained for either display orientation.

There is some freedom in implementing the invention.

The lens curvatures can be positive or negative, for example asexplained with reference to FIGS. 2 and 3.

In some configurations, a spacer can be integrated with a lens by makingthe planar side of the lens thicker.

Either one or both of the lenses could be made as a switchable lens, forinstance using one of the techniques that are described above. Thiscould be used to enable the lens function to be switched off completelyfor a 2D mode, or it could be used to enable parallax in one directiononly but with a higher resolution in another direction.

In a system with thick lenses and various refractive indexes, the aboverelations are only rough approximations. In practice, a balance will befound through numerical simulation and by choosing materials, lensshapes and spacer thicknesses in conjunction. These parameters aretypically optimized such that the viewing cone is similar in bothdirections (e.g. portrait and landscape).

It may be desired to decrease the total thickness of the structure toreduce weight and size for a portable device. For this reason, in apreferred embodiment it will be advantageous to realize the lower spacerwith a higher refractive index, whilst the top spacer should have alower refractive index, for example air. In this manner, the total stackthickness is reduced whilst maintaining the optical ratio (e.g. 3:1) tomaintain cone sizes. A further consequence of such an approach is thatthe lens interfaces will preferably have opposite curvatures.

Two example solutions will now be presented.

1. Air Gap Solution

This solution can have the structure as shown in FIG. 5. Spacer 22 isglass/plastic, for example with refractive index 1.5.

Lens 20 is glass/plastic and plano-convex as shown in FIG. 5.

Spacer 26 is an air gap with mechanical supports to provide the desiredfixed distance.

Lens 24 is glass/plastic and also plano-convex (as shown in FIG. 5).

FIG. 6 shows a simulation of the performance of the structure of FIG. 5,showing the illuminance on a detector plane placed at the optimalviewing distance from the display, with three views out of four turnedon. FIG. 6 shows that non-equal viewing cone distributions for theregular microlens (FIG. 4) changes to equal viewing cones. FIG. 6 issimilar to FIG. 4 and again shows the light power per unit area (inWatts per mm²) at different positions across the display screen. Oneplot is for the landscape mode and the other is for the portrait mode.

2. Low Refractive Index Difference Solution

This solution can have the structure shown in FIG. 7. In this context, alow refractive index is in the range 1.3-1.5 (typically 1.4), a highrefractive index is in the range 1.45-1.75 (typically 1.6), and a lowrefractive index difference is in the range 0.1-0.3 (typically 0.2).

Spacer 22 is glass/plastic with high refractive index. Lens 20 isintegrated with spacer 22 and is the same glass/plastic with the samehigh refractive index and is plano-convex.

The spacer 26 has a low refractive index. The lens/spacer unit 20,22 islaminated to the second spacer 26 with low index-matching glue.

The second lens 24 also has a high refractive index and is plano-convex,and is laminated to the spacer 26 with low index-matching glue. However,the second lens is inverted compared to the first lens, so that itdefines a concave lens shape with respect to the direction of displaylight through the lens stack. The first lens 20 is thus arranged asshown in FIG. 2 and the second lens 24 is arranged as shown in FIG. 3.

There can be more than two refractive index values in the system, buteach interface gives reflections that add to the 3D crosstalk.Unnecessary interfaces should thus be avoided.

The two examples above are based on the use of lenticular lenses. FIG. 8shows in schematic form an alternative approach in which the same designmethodology is applied to a barrier type display. A first barrier layer70 is over a first spacer layer (not shown) which is over the displaypanel 2, and the second barrier layer 72 is over the second spacer layer(not shown).

The spacing sizes are selected using the methodology above, with thebarrier opening widths and pitch dependent on the underlying pixelstructure, in the same way as for the lenticular designs.

The display panel typically has a sub-pixel grid with elongatedsub-pixels, for example as in the RGB stripe display. Elongatedsub-pixels are also used in other pixel configurations and the inventioncan be applied more generally.

The invention can be applied to phones, tablets and cameras withautostereoscopic displays.

The two view forming layers may have orthogonal lenticulars or barriers,but even for the portrait/landscape function, they may not beorthogonal. For example they may be vertical in one mode but slanted tothe vertical in the other mode. A typical slant is arctan(1/6)=9.46degrees. Thus, the lenticulars may be orthogonal or at 80.54 degrees forthis example of slant. Other slant angles are of course possible.

Other variations to the disclosed embodiments can be understood andeffected by those skilled in the art in practicing the claimedinvention, from a study of the drawings, the disclosure, and theappended claims. In the claims, the word “comprising” does not excludeother elements or steps, and the indefinite article “a” or “an” does notexclude a plurality. The mere fact that certain measures are recited inmutually different dependent claims does not indicate that a combinationof these measured cannot be used to advantage. Any reference signs inthe claims should not be construed as limiting the scope.

1. A multi-view display, comprising: a display panel; and a view formingarrangement formed over the display panel for providing a multi-viewfunction, wherein the view forming arrangement comprises a first viewforming structure spaced by a first distance (t1) from the display panelfor providing multiple views across a first direction, and a second viewforming structure spaced by a second distance (t2) from the first viewforming structure for providing multiple views across a secondperpendicular direction, characterised in that the angular width of themultiple views in the two directions is independently defined with theangular widths of the multiple views in the two directions in the ratioof smaller angular width to larger angular width of 1:n where n<2wherein the first view forming structure closest to the display panel ismade of material with a first refractive index, and the second viewforming structure is made of material with a lower refractive index. 2.A display as claimed in claim 1, wherein the view forming arrangementcomprises a first spacer layer over the display pane, a first lenticularlens array over the first spacer layer, a second spacer layer over thefirst lenticular lens array and a second lenticular lens array over thesecond spacer layer.
 3. A display as claimed in claim 2, wherein thefirst and second lenticular lens arrays define convex lens interfaces,with respect to the direction of light through the view formingarrangement from the display panel.
 4. A display as claimed in claim 3,wherein the first spacer layer, the first lenticular lens array and thesecond lenticular lens array are glass or plastic, and the second spacerlayer is air.
 5. A display as claimed in claim 2, wherein the firstlenticular lens array defines convex lens interfaces, and the secondlenticular lens array defines concave lens interfaces, with respect tothe direction of light through the view forming arrangement from thedisplay panel.
 6. A display as claimed in claim 5, wherein the firstspacer layer, the first lenticular lens array and the second lenticularlens array are glass or plastic with a first refractive index, and thesecond spacer layer is glass or plastic with a second, lower refractiveindex.
 7. A display as claimed in claim 1, wherein the view formingarrangement comprises a first spacer layer over the display panel, afirst barrier layer over the first spacer layer, a second spacer layerover the first barrier layer and a second barrier layer over the secondspacer layer.
 8. A hand held device comprising a display as claimed inclaim 1.